Automatic Detection of Interplanetary Coronal Mass Ejections from In Situ Data: A Deep Learning Approach
Gautier Nguyen, Nicolas Aunai, Dominique Fontaine, Erwan Le Pennec, Joris Van den Bossche, Alexis Jeandet, Brice Bakkali, Louis Vignoli, and Bruno Regaldo-Saint Blancard The Astrophysical Journal, Volume 874, Number 2 doi : https://doi.org/10.3847/1538-4357/ab0d24
Decades of studies have suggested several criteria to detect interplanetary coronal mass ejections (ICME) in time
series from in situ spacecraft measurements. Among them, the most common are an enhanced and smoothly
rotating magnetic field, a low proton temperature, and a low plasma beta. However, these features are not all
observed for each ICME due to their strong variability. Visual detection is time-consuming and biased by the
observer interpretation, leading to non-exhaustive, subjective, and thus hardly reproducible catalogs. Using
convolutional neural networks on sliding windows and peak detection, we provide a fast, automatic, and multiscale
detection of ICMEs. The method has been tested on the in situ data from WIND between 1997 and 2015, and
on the 657 ICMEs that were recorded during this period. The method offers an unambiguous visual proxy of
ICMEs that gives an interpretation of the data similar to what an expert observer would give. We found at a
maximum 197 of the 232 ICMEs of the 2010–2015 period (recall 84%±4.5%), including 90% of the ICMEs
present in the lists of Nieves-Chinchilla et al. and Chi et al. The minimal number of False Positives was 25 out of 158 predicted ICMEs (precision 84%±2.6%). Although less accurate, the method also works with one or several
missing input parameters. The method has the advantage of improving its performance by just increasing the
amount of input data. The generality of the method paves the way for automatic detection of many different event
signatures in spacecraft in situ measurements.
Signatures of Cold Ions in a Kinetic Simulation of the Reconnecting Magnetopause
Dargent, J., Aunai, N., Lavraud, B., Toledo‐Redondo, S., Califano, F. ( 2019). Journal of Geophysical Research: Space Physics, 124, 2497– 2514.
At the Earth’s magnetopause, a low‐energy ion population of ionospheric origin is commonly observed at the magnetospheric side. In this work we use a 2‐D fully kinetic simulation to identify several original signatures related to the dynamics of cold ions involved in magnetic reconnection at the asymmetric dayside magnetopause. We identify several original signatures of the cold ions dynamics driven by the development of magnetic reconnection at the asymmetric dayside magnetopause. We find that cold ions tend to rarefy in the diffusion region, while their density is enhanced as a result of compression along magnetospheric separatrices. We also observe the formation of crescent‐shaped cold ion distribution functions along the separatrices in the near‐exhaust region, and we present an analytical model to explain this signature. Finally, we give evidence of a localized parallel heating of cold ions. These signatures should be detected with the magnetospheric multiscale mission high‐resolution observations.
Analyzing the magnetopause internal structure: New possibilities offered by MMS tested in a case studys
Rezeau, L., Belmont, G., Manuzzo, R., Aunai, N., Dargent, J. ( 2018). Journal of Geophysical Research: Space Physics, 123, 227– 241. doi : https://doi.org/10.1002/2017JA024526
We explore the structure of the magnetopause using a crossing observed by the Magnetospheric Multiscale (MMS) spacecraft on 16 October 2015. Several methods (minimum variance analysis, BV method, and constant velocity analysis) are first applied to compute the normal to the magnetopause considered as a whole. The different results obtained are not identical, and we show that the whole boundary is not stationary and not planar, so that basic assumptions of these methods are not well satisfied. We then analyze more finely the internal structure for investigating the departures from planarity. Using the basic mathematical definition of what is a one‐dimensional physical problem, we introduce a new single spacecraft method, called LNA (local normal analysis) for determining the varying normal, and we compare the results so obtained with those coming from the multispacecraft minimum directional derivative (MDD) tool developed by Shi et al. (2005). This last method gives the dimensionality of the magnetic variations from multipoint measurements and also allows estimating the direction of the local normal when the variations are locally 1‐D. This study shows that the magnetopause does include approximate one‐dimensional substructures but also two‐ and three‐dimensional structures. It also shows that the dimensionality of the magnetic variations can differ from the variations of other fields so that, at some places, the magnetic field can have a 1‐D structure although all the plasma variations do not verify the properties of a global one‐dimensional problem. A generalization of the MDD tool is proposed.
Perpendicular Current Reduction Caused by Cold Ions of Ionospheric Origin in Magnetic Reconnection at the Magnetopause: Particle‐in‐Cell Simulations and Spacecraft Observations
Geophysical Research Letters, 45, 10,033– 10,042. doi : https://doi.org/10.1029/2018GL079051
Sergio Toledo‐Redondo Jérémy Dargent Nicolas Aunai Benoit Lavraud Mats André Wenya Li Barbara Giles Per‐Arne Lindqvist Robert E. Ergun Christopher T. Russell James L. Burch
Cold ions of ionospheric origin are present throughout the Earth’s magnetosphere, including the dayside magnetopause, where they modify the properties of magnetic reconnection, a major coupling mechanism at work between the magnetosheath and the magnetosphere. We present Magnetospheric MultiScale (MMS) spacecraft observations of the reconnecting magnetopause with different amounts of cold ions and show that their presence reduces the Hall term in the Ohm’s law. Then, we compare two particle‐in‐cell simulations, with and without cold ions on the magnetospheric side. The cold ions remain magnetized inside the magnetospheric separatrix region, leading to the reduction of the perpendicular currents associated with the Hall effect. Moreover, this reduction is proportional to the relative number density of cold ions. And finally, the Hall electric field peak is reduced along the magnetospheric separatrix owing to cold ions. This should have an effect on energy conversion by reconnection from electromagnetic fields to kinetic energy of the particles.
Smilei : A collaborative, open-source, multi-purpose particle-in-cell code for plasma simulation
Computer Physics Communications Volume 222, January 2018, Pages 351-373
DOI : https://doi.org/10.1016/j.cpc.2017.09.024
J.Derouillat, A. Beck, F. Pérez, T.Vinci, M. Chiaramello, A. Grassi, M.Flé, G.Bouchar, I. Plotnikov, N.Aunai, J.Dargent, C.Riconda, M.Grech
Smilei is a collaborative, open-source, object-oriented (C++) particle-in-cell code. To benefit from the latest advances in high-performance computing (HPC), Smilei is co-developed by both physicists and HPC experts. The code’s structures, capabilities, parallelization strategy and performances are discussed. Additional modules (e.g. to treat ionization or collisions), benchmarks and physics highlights are also presented. Multi-purpose and evolutive, Smilei is applied today to a wide range of physics studies, from relativistic laser–plasma interaction to astrophysical plasmas.
Kinetic simulation of asymmetric magnetic reconnection with cold ions
Journal of Geophysical Research: Space Physics, Volume 122, Issue 5, pp. 5290-5306 DOI : http://dx.doi.org/10.1002/2016JA023831
Dargent, J.; Aunai, N.; Lavraud, B.; Toledo-Redondo, S.; Shay, M. A.; Cassak, P. A.; Malakit, K.
At the dayside magnetopause, the magnetosphere often contains a cold ion population of ionospheric origin. This population is not always detectable by particle instruments due to its low energy, despite having an important contribution to the total ion density and therefore an impact on key plasma processes such as magnetic reconnection. The exact role and implications of this low-temperature population are still not well known and has not been addressed with numerical simulation before. We present 2-D fully kinetic simulations of asymmetric magnetic reconnection with and without a cold ion population on the magnetospheric side of the magnetopause, but sharing the same total density, temperature, and magnetic field profiles. The comparison of the simulations suggests that cold ions directly impact signatures recently suggested as a good marker of the X line region: the Larmor electric field. Our simulations reveal that this electric field, initially present all along the magnetospheric separatrix, is related to the bounce of magnetosheath ions at the magnetopause magnetic field reversal through Speiser-like orbits. Once reconnection widens the current sheet away from the X line, the bouncing stops and the electric field signature remains solely confined near the X line. When cold ions are present, however, their very low temperature enables them to E × B drift in the electric field structure. If their density is large enough compared to other ions, their contribution to the momentum equation is capable of maintaining the signature away from the X line. This effect must be taken into account when analyzing in situ spacecraft measurements.
Orientation of the X-line in asymmetric magnetic reconnection
Journal of Plasma Physics, Volume 82, Issue 4, article id. 535820401, 15 pp.DOI : http://dx.doi.org/10.1017/S0022377816000647
Aunai, N.; Hesse, M.; Lavraud, B.; Dargent, J.; Smets, R.
Magnetic reconnection can occur in current sheets separating magnetic fields sheared by any angle and of arbitrarily different amplitudes. In such asymmetric and non-coplanar systems, it is not yet understood what the orientation of the X-line will be. Studying how this orientation is determined locally by the reconnection process is important to understand systems such as the Earth magnetopause, where reconnection occurs in regions with large differences in upstream plasma and field properties. This study aims at determining what the local X-line orientation is for different upstream magnetic shear angles in an asymmetric set-up relevant to the Earth’s magnetopause. We use two-dimensional hybrid simulations and vary the simulation plane orientation with regard to the fixed magnetic field profile and search for the plane maximizing the reconnection rate. We find that the plane defined by the bisector of upstream fields maximizes the reconnection rate and this appears not to depend on the magnetic shear angle, domain size or upstream plasma and asymmetries.
Cold ion heating at the dayside magnetopause during magnetic reconnection
Geophysical Research Letters, Volume 43, Issue 1, pp. 58-66 DOI : http://dx.doi.org/10.1002/2015GL067187
Toledo-Redondo, S.; André, M.; Vaivads, A.; Khotyaintsev, Yu. V.; Lavraud, B.; Graham, D. B.; Divin, A.; Aunai, N.
Cold ions of ionospheric origin are known to be present in the magnetospheric side of the Earth’s magnetopause. They can be very abundant, with densities up to 100 cm-3. These cold ions can mass load the magnetosphere, changing global parameters of magnetic reconnection, like the Alfvén speed or the reconnection rate. In addition they introduce a new length scale related to their gyroradius and kinetic effects which must be accounted for. We report in situ observations of cold ion heating in the separatrix owing to time and space fluctuations of the electric field. When this occurs, the cold ions are preheated before crossing the Hall electric field barrier. However, when this mechanism is not present cold ions can be observed well inside the reconnection exhaust. Our observations suggest that the perpendicular cold ion heating is stronger close to the X line owing to waves and electric field gradients linked to the reconnection process.
Asymmetric kinetic equilibria: Generalization of the BAS model for rotating magnetic profile and non-zero electric field
Published in Physics of Plasmas DOI : http://dx.doi.org/10.1063/1.4930210
Nicolas Dorville, Gérard Belmont, Nicolas Aunai, Jérémy Dargent and Laurence Rezeau
Finding kinetic equilibria for non-collisional/collisionless tangential current layers is a key issue as well for their theoretical modeling as for our understanding of the processes that disturb them, such as tearing or Kelvin Helmholtz instabilities. The famous Harris equilibrium [E. Harris, Il Nuovo Cimento Ser. 10 23, 115–121 (1962)] assumes drifting Maxwellian distributions for ions and electrons, with constant temperatures and flow velocities; these assumptions lead to symmetric layers surrounded by vacuum. This strongly particular kind of layer is not suited for the general case: asymmetric boundaries between two media with different plasmas and different magnetic fields. The standard method for constructing more general kinetic equilibria consists in using Jeans theorem, which says that any function depending only on the Hamiltonian constants of motion is a solution to the steady Vlasov equation [P. J. Channell, Phys. Fluids (1958–1988) 19, 1541 (1976); M. Roth et al., Space Sci. Rev. 76, 251–317 (1996); and F. Mottez, Phys. Plasmas 10, 1541–1545 (2003)]. The inverse implication is however not true: when using the motion invariants as variables instead of the velocity components, the general stationary particle distributions keep on depending explicitly of the position, in addition to the implicit dependence introduced by these invariants. The standard approach therefore strongly restricts the class of solutions to the problem and probably does not select the most physically reasonable. The BAS (Belmont-Aunai-Smets) model [G. Belmont et al., Phys. Plasmas 19, 022108 (2012)] used for the first time the concept of particle accessibility to find new solutions: considering the case of a coplanar-antiparallel magnetic field configuration without electric field, asymmetric solutions could be found while the standard method can only lead to symmetric ones. These solutions were validated in a hybrid simulation [N. Aunai et al., Phys. Plasmas (1994-present) 20, 110702 (2013)], and more recently in a fully kinetic simulation as well [J. Dargent and N. Aunai, Phys. Plasmas (submitted)]. Nevertheless, in most asymmetric layers like the terrestrial magnetopause, one would indeed expect a magnetic field rotation from one direction to another without going through zero [J. Berchem and C. T. Russell, J. Geophys. Res. 87, 8139–8148 (1982)], and a non-zero normal electric field. In this paper, we propose the corresponding generalization: in the model presented, the profiles can be freely imposed for the magnetic field rotation (although restricted to a 180 rotation hitherto) and for the normal electric field. As it was done previously, the equilibrium is tested with a hybrid simulation.
On the electron diffusion region in planar, asymmetric, systems
Published in Geophysical Research Letters DOI : http://dx.doi.org/10.1002/2014GL061586
Hesse, Michael; Aunai, Nicolas; Sibeck, David; Birn, Joachim
simulations and analytical theory are employed to study the electron diffusion region in asymmetric reconnection, which is taking place in planar configurations without a guide field. The analysis presented here focuses on the nature of the local reconnection electric field and on differences from symmetric configurations. Further emphasis is on the complex structure of the electron distribution in the diffusion region, which is generated by the mixing of particles from different sources. We find that the electric field component that is directly responsible for flux transport is provided not by electron pressure-based, “quasi-viscous,” terms but by inertial terms. The quasi-viscous component is shown to be critical in that it is necessary to sustain the required overall electric field pattern in the immediate neighborhood of the reconnection X line.